Active implantable medical devices typically have a generator that has a metal housing, typically made of titanium, on which a connector head is mounted. The connector head, also referred to as simply a connector, is provided to mechanically and electrically connect the generator to one or more leads. The leads have at their distal end various electrodes used for sensing electrical activity signals of the patient and delivering stimulation pulses, e.g., pacing and defibrillation to the patient's heart.
The connection of the connector head to various electronic circuits enclosed in the generator housing involves establishing several feedthroughs in the housing. Each of the feedthroughs has a pin conductor to be connected to a corresponding plug of the connector head at its one end emerging from the upper surface of the housing (i.e., on the outer or external side), and to be connected to the electronics at its other end emerging from the surface of the housing opening into the interior volume of the housing (i. e., on the inner or internal side) where the electronic circuits are located.
One type of feedthrough structure is described, for example, in EP 1177815 A1 and its counterpart U.S. Pat. No. 6,574,508 (both assigned to Sorin CRM S.A.S, previously known as ELA Medical). In addition to the connection pins on the connector head, other feedthroughs are provided, for example, to provide a connection with a surface electrode placed on the outside of the housing.
These feedthroughs can also be found in other sub-components of active medical devices such as batteries and capacitors.
It is important that a feedthrough must electrically isolate the particular conductor passing through the metal housing and also form an hermetic seal to prevent penetration of body fluids into the housing, throughout the life of the implanted device, typically for ten years. Therefore, a feedthrough is a key element in design of the housing of active implantable devices, particularly defibrillators and pacemakers, for providing a dual function of electrical current flow and of sealing of the housing.
For electrical isolation, dielectric materials such as ceramic or glass are used to form highly resistant mechanical connections with the metal of the housing. To this end, the insulating material of the feedthrough is surrounded by a metal collar and welded to the housing of the device, or to the half-housing prior to two half housings being joined together. The connections between the pins and the insulating material are made hermetic by brazing with a suitable material, such as gold, in the interface area to ensure complete sealing and insulation.
The complexity of the formation of these feedthroughs and the use of specific components and technologies are partly attributed to the high price of the feedthroughs, which may contribute up to 10% of the total cost of a device. In addition, the current trend of increasing the number of electrodes (e.g., in “multisite” devices) requires an increase in the number of contacts on the connector, and the number of pins, consequently increasing the cost of forming of the feedthroughs.
Other challenges remain with existing techniques for forming feedthroughs. In particular, due to the fact that the feedthrough is integrated into the housing, the hermetic seal of the housing that is made of titanium, is generally performed by laser welding, which is a complex and costly operation because it may introduce gaps, both vertically and horizontally, sometimes too large for the laser welding to be effective.
Furthermore, when integrated to the housing, the feedthroughs may undergo significant thermomechanical stresses during laser welding due to its heterogeneous structure and present reliability and reworking concerns.
Furthermore, feedthroughs require a grouping of electric wires, therefore they must be sized appropriately to accommodate the wire pins. As these electrical wires are to be connected to the connector head at points of contact that are spaced apart, it is necessary to impose relatively large curvatures to the electrical pins coming out of the feedthrough, to direct the wire pins to the appropriate points of contact on the connector head.
These important roles and features of feedthroughs inside and outside of the housing severely limit the degree of miniaturization during the device design.
Although the currently known feedthroughs adequately fulfill their intended functions (e.g., hermeticity and transfer of the electrical signals), they contribute a large share of manufacturing cost to implantable medical devices, which is not only a constraint in the design of such devices but also a limitation to miniaturization and a significant source of reliability defects during the assembly of the housing. These difficulties have not yet been appropriately overcome.
Various alternative techniques have been proposed, but they are relatively complex and have drawbacks.
U.S. Pat. Publication No. 2007/0112396 A1 describes a technique for forming a feedthrough for an implantable medical device such as a cochlear prosthesis. The feedthrough is made to be conductive by locally doping silicon on selected paths, instead of making a driver (pin) to extend from one side of the feedthrough. One difficulty with this structure lies in the absence of physical isolation of the area of electrical conduction, because the interface between doped silicon and undoped silicon is not completely insulated. In addition, this technique requires the transfer of a silicon wafer to the device housing to manufacture the feedthrough. It requires silicon/titanium brazing that has roughly the same drawbacks and difficulties of the ceramic/titanium brazing technique as mentioned above.
U.S. Pat. Publication No. 2007/0060969 A1 proposes to make the feedthrough by stacking a number of layers of green ceramic provided with staggered vias that are interconnected by a conductive adhesive applied between the number of layers. However, this technique has the same drawbacks as the previous technique lacking physical separation between insulating and conductive materials (although in this case, the materials are different), and requiring an assembly of the ceramic feedthrough on the titanium case by a hermetic brazing. In addition, the manufacturing cost is relatively high because of the relatively complex operations and the use of ceramics and conductive materials of platinum/iridium or gold.
In areas (including aeronautics) other than medical implants, other feedthrough techniques have been proposed, for example, as described in U.S. Pat. Nos. 5,166,097 A and 5,322,816 A. But these feedthroughs are made from a silicon wafer that is applied later on the metal housing with technological constraints and difficulties involved in such an operation.